The present invention relates to an ink jet recording method and an ink jet recording device discharging ink by electrostatic force, and more particularly to an ink jet recording method and device controlling the formation of a meniscus around a leading edge of a projection when ink is discharged from the leading edge of the projection.
In ink jet recording systems, processing such as a development stage is not required, and recording heads are small in size so that recording devices can be easily miniaturized. Accordingly, the ink jet recording systems have widely come in practice.
The conventional ink jet recording systems include a system using piezoelectric elements deformed depending on electric signals, a system using heating resistors generating heat depending on electric signals, and a system using electrostatic force according to electric signals.
All the above-mentioned ink jet recording systems have the problem that poor image drawing occurs by an increase in ink viscosity or solidification of ink caused by evaporation of solvent for ink from ink discharge portions Accordingly, ink jet recording devices are equipped with means for sealing ink discharge outlets when printing is not conducted, and means for cleaning the discharge outlets, if desired,
With respect to such a problem, JP-A-11-192732 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a method of discharging ink for every predetermined period from a recording head independently of image drawing for preventing clogging of ink, and JP-A-2000-127417 discloses a technique relating to a method for cleaning a discharge outlet. For solving such a problem, ink used in such ink jet recording systems contains an aqueous or organic solvent in a large amount, in which dye or pigment is contained as a coloring agent, and is low in viscosity and low in the concentration of the coloring agent.
In such ink jet recording systems, therefore, blurs of images occur so that it is difficult to form images of high image quality and high resolution. Further, the time is required for drying image formation areas so that it is difficult to improve the productivity.
On the other hand, for achieving an ink jet recording system giving reduced blurs of images, high drying speed, and high image quality and good productivity, Japanese Patent No. 3000672 and JP-A-2000-63723 disclose ink in which particles of coloring agent are dispersed in a solvent. For reducing the blurs of images and increasing the drying speed in the above-mentioned ink, it is effective to increase the coloring agent particle concentration and decrease the solvent concentration.
However, the application of the ink increased in the coloring agent particle concentration and decreased in the solvent concentration to the conventional ink jet recording systems results in clogging of the ink in a discharge portion. It is therefore difficult to draw good images. In particular, when minute droplets are discharged for obtaining images of high image quality and high resolution in the system using piezoelectric elements deformed depending on electric signals and the system using heating resistors generating heat depending on electric signals, it is necessary to decrease a nozzle diameter of the discharge portion, which is liable to cause clogging in the discharge portion.
Further, in the ink jet recording system in which ink is discharged by electrostatic force, a high-voltage pulse electric signal is necessary for discharging the ink increased in the coloring agent particle concentration and decreased in the solvent concentration. As a result, another problem arises that control units become extremely expensive.
On the other hand, as disclosed in International Patent Publication No. 501490/1998, a recording device is proposed in which ink in an ink discharge outlet of a recording head is kept in a state just before discharge, energy is given to the ink in the ink discharge outlet by an image signal through a meniscus formation unit (a unit for giving energy to ink such as a heater, an ultrasonic generator or a piezoelectric element) in response to an image signal, thereby destroying the balance of the ink which has been in a balanced state between electrostatic force and surface tension up to then to discharge an ink droplet from the ink discharge outlet, and the ink droplet is allowed to travel toward a recording medium while being accelerated by the electrostatic force to make a recording.
FIG. 5(a)-(1) is a schematic cross sectional view showing a recording device based on such a principle, and FIG. 5 (a)-(2) is a schematic plan view thereof. In the schematic plan view, a second bias electrode, an ink meniscus formation unit and an image receiving sheet are excluded for convenience"" sake.
Referring to FIGS. 5(a)-(1) and 5(a)-(2), numeral 1xe2x80x2 is a recording head, numeral 10xe2x80x2 is an ink chamber, numeral 11xe2x80x2 is a discharge outlet, numeral 13 is an ink meniscus formation unit, numeral 141 is a first bias electrode, and numeral 142 is a second bias electrode. Numeral 15 is a meniscus control unit for controlling the meniscus formation unit 13, numeral 16 is a discharge control unit for controlling an electric signal applied across the first bias electrode 141 and the second bias electrode 142, numeral 20 is an image receiving sheet fixed to an image receiving sheet fixing member (not shown) and moving in the direction indicated by the arrow, and numeral 9 is ink.
In the recording device, the discharge control unit 16 has previously applied a bias voltage Vb across the first bias electrode 141 and the second bias electrode 142. The bias voltage Vb is such a voltage that the ink in the ink discharge outlet 11xe2x80x2 of the recording head 1xe2x80x2 is not discharged.
Then, energy is given to the ink in the ink discharge outlet 11xe2x80x2 by the meniscus formation unit 13 in response to an image signal, thereby destroying the balance of the ink which has been in a balanced state between electrostatic force and surface tension up to then to discharge an ink droplet from the ink discharge outlet 11xe2x80x2.
The discharged ink droplet is allowed to travel toward the image receiving sheet (recording medium) 20 while being accelerated by an electric field formed between the first bias electrode 141 and the second bias electrode 142 to make a record on the recording medium 20.
FIGS. 5(b)-1 to 5(b)-5 show the principle of printing operation as described above.
An electrically heated transducer is used as the meniscus formation unit, and the ink in the discharge outlet is heated thereby to elevate the temperature thereof at the meniscus, thus forming an ink droplet. When the temperature is elevated, the surface tension is lowered below the critical surface tension. As a result, the ink is discharged from the discharge outlet. The ink droplet discharged is accelerated in the direction of the bias electrode 142, and collides with the recording medium 20.
The radius of the discharge outlet used herein is 20 xcexcm.
FIG. 5(b)-1 is a cross sectional view showing the discharge outlet at a standstill position, and the ink is pressurized by the bias voltage. As a result, the ink meniscus expands, and the expansion of the ink meniscus allows the electric field to slightly concentrate. Bonding force by the ink pressure and the electric field is in a state of equilibrium with the surface tension.
FIG. 5(b)-2 shows a nozzle just after an energy supply pulse has been supplied to the meniscus formation unit (electrically heated transducer) 13. Heat is transmitted to a surface of the ink, and the resulting increase in temperature causes a local decrease in the surface tension of the ink to somewhat develop the ink meniscus.
FIG. 5(b)-3 shows further development of the ink meniscus. The ink meniscus exhibits a substantially cylindrical form by a gradient of the surface tension from the discharge outlet to the center of the meniscus. In this stage, almost all movements of the ink are still caused by positive ink pressure. However, the electric field acting on the ink becomes strong enough to attract the ink from the nozzle.
FIG. 5(b)-4 shows development of the ink meniscus a little later after the electrically heated transducer has been turned off, The surface tension starts to increase, and the ink starts to return to the discharge outlet. The ink at a tip of the ink meniscus is still attracted in the direction of the recording medium so that the ink meniscus starts to be constricted.
FIG. 5(b)-5 shows the ink droplet after it has been separated from the ink itself. Although the ink droplet is partially polarized in the electric field, it still has some charges. Accordingly, the ink droplet is accelerated in the direction of the bias electrode to collide with the recording medium.
Thus, a simple constant-voltage power supply can be used for generating the electric field, and it is unnecessary to separate the electric field applied to the nozzle from an electric field applied to an adjacent nozzle. It is therefore possible to narrow the space between the nozzles.
However, the recording device is subjected to the restriction that the discharge outlet cannot be made very larger for forming the strong electric field in the vicinity of the discharge outlet. Further, for forming a minute dot, the radius of the discharge outlet used herein is 20 xcexcm. Accordingly, the use of high-viscosity ink increased in the coloring agent particle concentration and decreased in the solvent concentration in the recording device having such a narrow discharge outlet results in clogging of the ink in the discharge outlet. It has been therefore impossible to use such high-concentration and high-viscosity ink. For achieving no appearance of ink blurs on the recording medium, rapid drying speed and recording of high image quality and good productivity, it is disadvantageous that such high-concentration and high-viscosity ink cannot be used.
Giving attention to the above-mentioned problem, the invention has been made.
It is therefore an object of the invention to provide an ink jet recording method which gives reduced blurs, high drying speed, and high image quality and good productivity.
Another object of the invention is to provide an ink jet recording device discharging ink by electrostatic force, which gives reduced blurs, high drying speed, and high image quality and productivity, and is inexpensive.
According to the invention, there are provided:
1. An ink jet recording method comprising discharging ink from a leading edge of a projection to form an image, in which (1) the ink is discharged by applying a voltage across electrodes based on an image data signal, and forming a meniscus around the leading edge of the projection in synchronism with the image data signal, or (2) the ink is discharged by forming a meniscus around the leading edge of the projection based on an image data signal in an electrostatic field;
2. An inkjet recording device discharging an ink droplet toward an image receiving sheet, which comprises an ink tank and a recording head communicating with the ink tank, wherein the recording head comprises an ink chamber provided with a discharge outlet, an projection disposed at an approximately center portion in the ink chamber so that a leading edge thereof points toward the discharge outlet, a meniscus formation unit for forming an ink meniscus around the leading edge of the projection disposed in the ink chamber, a first discharge electrode arranged in the vicinity of the projection, a second discharge electrode arranged on the back side of the image receiving sheet, a discharge control unit for controlling an electric signal applied across the first discharge electrode and the second discharge electrode, and a meniscus control unit for controlling the meniscus formation unit, wherein the discharge control unit controls the electric signal comprising a pulse voltage based on an image data signal and a bias voltage irrespective of the image data signal, and the meniscus control unit controls the meniscus formation unit in synchronism with the image data signal;
3. An ink jet recording device discharging an ink droplet toward an image receiving sheet, which comprises an ink tank and a recording head communicating with the ink tank, wherein the recording head comprises an ink chamber provided with a discharge outlet, an projection disposed at an approximately center portion in the ink chamber so that a leading edge thereof points toward the discharge outlet, a meniscus formation unit for forming an ink meniscus around the leading edge of the projection disposed in the ink chamber, a first discharge electrode arranged in the vicinity of the projection, a second discharge electrode arranged on the back side of the image receiving sheet, a discharge control unit for controlling an electric-signal applied across the first discharge electrode and the second discharge electrode, a meniscus control unit for controlling the meniscus formation unit, a charge unit for charging the image receiving sheet and a charge control unit for controlling a charge amount of the charge unit, wherein the discharge control unit controls a pulse voltage based on an image data signal, the charge control unit controls a charge amount on the image receiving sheet to an amount corresponding to a bias voltage irrespective of the image data signal, and the meniscus control unit controls the meniscus formation unit in synchronism with the image data signal;
4. An ink jet recording device discharging an ink droplet toward an image receiving sheet, which comprises an ink tank and a recording head communicating with the ink tank, wherein the recording head comprises an ink chamber provided with a discharge outlet, an projection disposed at an approximately center portion in the ink chamber so that a leading edge thereof points toward the discharge outlet, a meniscus formation unit for forming an ink meniscus around the leading edge of the projection disposed in the ink chamber, a first bias electrode arranged in the vicinity of the projection, a second bias electrode arranged on the back side of the image receiving sheet, a bias voltage control unit for controlling A bias voltage applied across the first bias electrode and the second bias electrode, and a meniscus control unit for controlling the meniscus formation unit, wherein the bias voltage control unit controls the bias voltage irrespective of an image data signal, and the meniscus control unit controls the meniscus formation unit based on the image data signal;
5. An ink jet recording device discharging an ink droplet toward an image receiving sheet, which comprises an ink tank and a recording head communicating with the ink tank, wherein the recording head comprises an ink chamber provided with a discharge outlet, an projection disposed at an approximately center portion in the ink chamber so that a leading edge thereof points toward the discharge outlet, a meniscus formation unit for forming an ink meniscus around the leading edge of the projection disposed in the ink chamber, a meniscus control unit for controlling the meniscus formation unit, a charge unit for charging the image receiving sheet and a charge control unit for controlling a charge amount of the charge unit, wherein the charge control unit controls a charge amount on the image receiving sheet to an amount corresponding to a bias voltage irrespective of the image data signal, and the meniscus control unit controls the meniscus formation unit based on the image data signal;
6. The ink jet recording device described in any one of items 2 to 5, wherein the meniscus formation unit is any one of a piezoelectric element, a heating element and an ultrasonic generating element, or a combination of two or more thereof;
7. The ink jet recording device described in any one of items 2 to 6, wherein the projection has a dielectric constant of 3 or more;
8. The ink jet recording device described in any one of items 2 to 7, wherein the ink tank is communicated with the ink charter through a porous member;
9. The ink jet recording device described in any one of items 2 to 8, wherein the recording head further comprises a temperature detecting unit for detecting the temperature of the recording head, and a temperature control unit for heating and/or cooling the recording head in response to the temperature detected;
10. The ink jet recording device described in any one of items 2 to 8, which further comprises a first accelerative electrode provided in the vicinity of the recording head, and a second accelerative electrode provided on the back side of the image receiving sheet;
11. An recording head comprising an ink chamber provided with a discharge outlet, an projection disposed at an approximately center portion in the ink chamber so that a leading edge thereof points toward the discharge outlet, a meniscus formation unit for forming an ink meniscus around the leading edge of the projection disposed in the ink chamber, a first discharge electrode arranged in the vicinity of the projection, a discharge control unit for controlling an electric signal applied across a second discharge electrode provided on the back side of an image receiving sheet for receiving an ink droplet discharged and the first discharge electrode, and a meniscus control unit for controlling the meniscus formation unit, wherein the discharge control unit controls the electric signal comprising a pulse voltage based on an image data signal and a bias voltage irrespective of the image data signal, and the meniscus control unit controls the meniscus formation unit in synchronism with the image data signal;
12. A recording head comprising an ink chamber provided with a discharge outlet, an projection disposed at an approximately center portion in the ink chamber so that a leading edge thereof points toward the discharge outlet, a meniscus formation unit for forming an ink meniscus around the leading edge of the projection disposed in the ink chamber, a first discharge electrode arranged in the vicinity of the projection, a discharge control unit for controlling an electric signal applied across a second discharge electrode provided on the back side of an image receiving sheet for receiving an ink droplet discharged and the first discharge electrode, and a meniscus control unit for controlling the meniscus formation unit, a charge unit for charging the image receiving sheet and a charge control unit for controlling a charge amount of the charge unit, wherein the discharge control unit controls a pulse voltage based on an image data signal, the charge control unit controls a charge amount on the image receiving sheet to an amount corresponding to a bias voltage irrespective of the image data signal, and the meniscus control unit controls the meniscus formation unit in synchronism with the image data signal;
13. An recording head comprising an ink chamber provided with a discharge outlet, an projection disposed at an approximately center portion in the ink chamber so that a leading edge thereof points toward the discharge outlet, a meniscus formation unit for forming an ink meniscus around the leading edge of the projection disposed in the ink chamber, a first bias electrode arranged in the vicinity of the projection, a bias voltage control unit for controlling a bias voltage applied across a second bias electrode provided on the back side of an image receiving sheet for receiving an ink droplet discharged and the first bias electrode, and a meniscus control unit for controlling the meniscus formation unit, wherein the bias voltage control unit controls the bias voltage irrespective of an image data signal, and the meniscus control unit controls the meniscus formation unit based on the image data signal;
14. A recording head comprising an ink chamber provided with a discharge outlet, an projection disposed at an approximately center portion in the ink chamber so that a leading edge thereof points toward the discharge outlet, a meniscus formation unit for forming an ink meniscus around the leading edge of the projection disposed in the ink chamber, a meniscus control unit for controlling the meniscus formation unit, a charge unit for charging the image receiving sheet and a charge control unit for controlling a charge amount of the charge unit, wherein the charge control unit controls a charge amount on the image receiving sheet to an amount corresponding to a bias voltage irrespective of the image data signal, and the meniscus control unit controls the meniscus formation unit based on the image data signal.
In the ink jet recording method of the invention, the ink is basically discharged from the leading edge of the projection to form the image, and 1) the ink is discharged by applying the voltage across the electrodes based on the image data signal, and forming a meniscus around the leading edge of the projection in synchronism with the image data signal (hereinafter referred to as aspect A), or 2) the ink is discharged by forming the meniscus around the leading edge of the projection based on the image data signal in the electrostatic field (hereinafter referred to as aspect B).
The electrostatic field in aspect B is formed in the extent sufficient for providing an electric field at the leading edge of the projection so that discharge of ink may occur upon the formation of ink meniscus around the leading edge of the projection. The electric field at the leading edge of the projection does not act to discharge the ink when the leading edge of the projection protrudes beyond the ink surface. The electrostatic field strength may be appropriately determined taking the shape and material of the projection into consideration.
According to aspect A of the invention, the ink jet recording device is provided with the ink chamber, the projection disposed at the approximately center portion in the ink chamber so that the leading edge thereof points toward the discharge outlets the first discharge electrode arranged in the vicinity of the projection, and the second discharge electrode arranged on the back side of the image receiving sheet for receiving the ink droplet discharged, the pulse voltage based on the image data signal and the bias voltage irrespective of the image data signal are applied across the discharge electrodes, or the pulse voltage based on the image data signal is applied across the discharge electrodes while the image receiving sheet is charged in an amount corresponding to the bias voltage irrespective of the image data signal, the ink meniscus is formed around the leading edge of the projection in synchronism with the image data signal, thereby discharging the ink. Accordingly, even when the bias voltage or the charge amount is increased, undesirable ink discharge does not occur. The pulse voltage can therefore be decreased. Further, since the ink is discharged from the leading edge of the projection, minute droplets can be discharged even when the discharge outlet is increased in size.
Furthermore, even the application of high-concentration and high-viscosity ink causes no clogging of the ink in the discharge outlet, because of the discharge outlet increased in size. Accordingly, it becomes possible to provide the ink jet recording device giving reduced blurs, high drying speed, high image quality and good productivity, at low cost.
The bias voltage in aspect A is applied in an amount sufficient for providing an electric field at the leading edge of the projection so that discharge of ink may not occur even when the ink meniscus is formed around the leading edge of the projection. The discharge of ink does not occurs until the electric field strength at the leading edge of the projection reaches to an amount capable of discharging the ink by means of the application of the pulse voltage.
According to aspect B of the invention, the bias voltage irrespective of the image data signal has been applied across the bias electrodes or the charge corresponding to the bias voltage irrespective of the image data signal has been provided on the image receiving sheet, and the meniscus formation unit forms the meniscus around the leading edge of the projection by the image signal, thereby discharging the ink. Accordingly, even when the bias voltage or the charge amount is increased, undesirable ink discharge does not occur. Further, since the ink is discharged from the leading edge of the projection, minute droplets can be discharged, even when the discharge outlet is increased in size.
Furthermore, even the application of high-concentration and high-viscosity ink causes no clogging of the ink in the discharge outlet, because of the discharge outlet increased in size. Accordingly, it becomes possible to provide the ink jet recording device giving reduced blurs, high drying speed, high image quality and good productivity, at low cost.
In aspect B, the bias voltage is applied in an amount sufficient for providing an electric field at the leading edge of the projection so that discharge of ink may occur upon the formation of ink meniscus around the leading edge of the projection. The electric field strength at the leading edge is sufficient for discharging the ink, but the discharge of ink does not occur until the meniscus is formed around the leading edge of the projection.